System and method for abandoning and recovering pipeline

ABSTRACT

A method of abandoning a pipeline ( 2 ) being laid by a vessel having an A/R system, wherein a sealine ( 2, 11, 13 ) is initially held by a pipe laying and tensioning arrangement. The method includes a step of reducing tension at sea level of the sealine ( 2, 11, 13 ), by connecting one or more buoyant members ( 13 ) to the end of the pipeline ( 2 ). The members ( 13 ) are then lowered into the sea, using a line ( 16 ). At least one member ( 13 ) is connected to the pipeline ( 2 ) through a connection ( 12 ) which allows pivoting of the member ( 13 ) so that the end of the member ( 13 ) at the end of the pipeline ( 2 ) projects upwardly from the seabed ( 3 ).

BACKGROUND OF THE INVENTION

This invention relates generally to the laying of pipelines and inparticular to a method for abandoning (or abandoning and recovering) apipeline. The invention also relates to a system for use in such amethod. Such a system and such a method are referred to herein as an A/Rsystem and an A/R method.

During laying, it is sometimes necessary to abandon a pipeline andrecover it later. For example, a pipeline might need to be abandonedbecause of severe weather conditions or mechanical problems aboard thelaying vessel.

Conventionally, such abandonment and recovery is carried out by a methodsuch as: welding an abandonment/recovery head onto the end of thepipeline being laid; connecting a steel rope to the head; transferringpipeline tension from the pipeline tensioning arrangement on the layingvessel that is used during normal laying to a winch on the vessel; andlaying the pipeline and head on the sea bed. In severe weatherconditions the vessel may also have to disconnect the rope from thewinch and abandon the rope for later recovery; the end of the rope maythen be marked by a buoy connected to it directly or via a pennant line.

Use of a conventional A/R system in deep water causes several problems.

The steel rope associated with the winch must have a very high tensilestrength to support the high load resulting from the long length ofpipeline, which initially extends from the seabed to the vessel. Thatrequires a steel rope having a larger diameter, which, in turn,increases its weight. A higher load capacity is then required of thewinch. The problem is increased because a very long steel rope and avery long pennant line are required, which creates further weight. Thefactors just described mean that the provision of a conventionallydesigned A/R system on a vessel able to operate in deep water is undulyexpensive.

OBJECTS AND SUMMARY

The effect of the factors referred to above is to make the provision ofan effective and economical A/R system problematic when pipelines arebeing laid in deep water.

It is an object of the invention to provide a method and apparatus oflaying a pipeline which overcomes or mitigates the above-mentionedproblem.

According to a first aspect of the present invention there is provided amethod of abandoning a pipeline being laid by a vessel, including thesteps of reducing the tension at sea level of the sealine being laid bythe vessel, and thereafter lowering the end of the sealine towards theseabed using a line from a winch on the vessel. Preferably, the tensionis reduced by at least 15%. Advantageously, the tension is reduced by atleast 30%, and more preferably by at least 40%.

By reducing the tension at sea level of the sealine being laid by thevessel before the winch is used to lower the sealine, it becomespossible to employ a winch whose maximum load bearing capacity issubstantially less than the maximum tension under which the vessel isdesigned to lay the pipeline (the tension when the pipeline is beinglaid at the greatest depth). The maximum load bearing capacity of thewinch may therefore be less than the maximum load bearing capacity ofthe pipe laying and tensioning arrangement that is used for laying thepipeline. Similarly the load bearing capacity of the winch line can bereduced thereby enabling its diameter to be reduced. In an example ofthe invention the load bearing capacity of the winch is reduced to about50% of the maximum tension under which the vessel is designed to lay apipeline. A reduction in the load bearing capacity (SWL) of the winch iscritical in enabling an effective and economical A/R system to beprovided even on a vessel designed to be capable of laying largediameter pipelines in deep water.

Where reference is made in the specification to a “sealine” it should beunderstood that the term is referring to the pipeline laid on the seabedtogether with any pipeline or other elongate members extending upwardlyfrom the pipeline.

In embodiments of the invention described below, the step of reducingthe tension at sea level of the sealine being laid by the vesselcomprises connecting one or more light elongate members to the end ofthe pipeline and lowering the elongate members into the sea. Wherereference is made to a “light” elongate member, it should be understoodthat the member is to be light per unit length, when submerged in water,by comparison with the weight in water of the pipeline to which it isconnected, so that the effect of connecting the light elongate member inthe sealine is to reduce the weight of the sealine. It will beappreciated that any reduction in weight of the sealine extendingbetween the vessel and the seabed will reduce the tension in the sealineat sea level. In the case where the pipeline is flooded, connection of afurther length of pipeline sealed at both ends to prevent floodingrepresents connection of a light elongate member and reduces the tensionin the sealine. Provided the water in which the method is carried out isnot too deep, the use of such sealed pipeline or similar members mayalone reduce the tension to a level that the winch is able to bear. Indeeper water, however, even when the pipeline is not flooded, thetension in the pipeline may be too great for the winch to bear; in thiscase connecting a length of sealed pipeline will not reduce the tensionand a lighter member is required to be connected; preferably such amember is sufficiently light that it is buoyant and, more preferably,generates an upward buoyancy force in water of the order of one half itsweight in air or more. In especially deep water it may be desirable toconnect together a series of buoyant members.

Said at least one buoyant elongate member may be composed principally ofa lightweight material, for example, one selected from the groupcomprising glass fibre reinforced resins, other composite materials,carbon, titanium and aluminium. In an embodiment of the inventiondescribed below the material is a glass fibre reinforced vinyl esterresin. Parts of the elongate member, for example, end fittings, may bemade of heavier material, for example, steel.

An intermediate elongate member is preferably connected between the endof the pipeline and a buoyant elongate member. The intermediate elongatemember is preferably hollow and sealed against the ingress of water. Theintermediate elongate member may be of sufficient weight not to bebuoyant when empty. Preferably the member is provided with a valve toenable the interior of the intermediate elongate member to be flooded bysea water. Especially in the event that the sealine cannot be recoveredby connecting a winch line to its end, the method may further includethe step of cutting the intermediate elongate member, connecting thewinch line to the end of the elongate member that is connected to thepipeline and recovering the end of the pipeline. The ability to floodthe interior of the intermediate elongate member enables pressures to beequalised, thereby facilitating the cutting of the member.

When the one or more elongate members first begin to be lowered into thesea the tension that has to be applied to them is substantially the sameas when laying pipe. It is a much preferred feature of the inventionthat the one or more elongate members are lowered using a pipe layingand tensioning arrangement on the vessel that is used for laying thepipeline; usually there is only one such tensioning arrangement althoughit may have several component parts. In order to facilitate use of thesame tensioning arrangement, it is preferred that the diameter of thelight elongate members is substantially the same as the diameter of theordinary lengths of pipe making up the pipeline. Indeed the lightelongate members are preferably of the same general dimensions as anordinary length of pipe. As will now be understood, the inventionenables the load bearing capacity of the winch to be substantially lessthan the load bearing capacity of the tensioning arrangement.

In some cases there will be no need to disconnect the winch line fromthe sealine because the laying vessel is able to remain at its station,connected via the winch line to the pipeline. In other cases, however,it will be desirable for the vessel to become completely freed and themethod of the invention therefore preferably includes the subsequentstep of disconnecting the sealine from the winch line. The disconnectioncan be effected in a variety of ways. A first possibility, if an ROV isprovided as would normally be the case, is for the disconnection of thesealine from the winch line to be initiated by a physical contact of theROV with a connector connecting the sealine to the winch line. It isalso possible for disconnection to be initiated remotely in other ways,for example by a signal transmitted from an ROV or the vessel; thesignal may be an acoustic signal. In that case the disconnection can beeffected without the need for any physical interaction with an ROV orany other outside agency. In one advantageous embodiment of theinvention the disconnection takes place automatically at a predeterminedangle of the adjoining ends of the winch line and the sealine attachedthereto. That predetermined angle is preferably in the range of 45° to75° and in an embodiment of the invention is about 60°.

Even in a case where the disconnection is initiated by physical contactfrom the ROV, it is preferable that the physical contact amounts merelyto a signal and that disconnection is able to take place with little orno intervention by the ROV or any other physical manipulation.Preferably the energy required for disconnection is stored in theconnecting arrangement.

Preferably said at least one elongate member is connected to thepipeline through a connection which allows pivoting of the elongatemember about a horizontal axis when the pipeline is lying horizontally.Such an arrangement also makes it easier to adopt the preferred methodin which, after disconnection of the sealine from the winch line, an endportion of the sealine projects upwards from the seabed.

Thus, according to a second aspect of the invention, there is provided amethod of abandoning a pipeline being laid by a vessel, including thesteps of connecting one or more buoyant elongate members to the end ofthe pipeline, thereafter lowering the elongate members into the sea andthen abandoning the pipeline, wherein one or more elongate members areso connected to the pipeline that the end of the elongate member at theend of the pipeline projects upwardly from the seabed, therebyfacilitating the future recovery of the pipeline. Preferably, at leastone elongate member is connected to the pipeline through a connectionwhich allows pivoting of the elongate member about a horizontal axiswhen the pipeline is lying horizontally. Preferably, there is also astep in which the abandoned pipeline is recovered, for example, asdescribed below with reference to the method according to the firstaspect of the invention.

The method of the first aspect of the invention also preferably providesfor recovery of the pipeline after abandonment. Thus the methodpreferably further includes recovering the abandoned pipeline bycarrying out the further steps of raising the end of the sealine to thevessel using the winch, and raising the sealine further using a pipelaying and tensioning arrangement on the vessel that is used for layingthe pipeline. As will be appreciated, in a case where the methodincludes disconnection of the winch line from the sealine, it ispreferred that the further steps also include a preliminary step ofconnecting the winch line to the sealine.

The invention further provides an A/R system for carrying out a methodof abandoning a pipeline as defined above with reference to either thefirst or second aspects of the invention. Such a system preferablyincludes a winch, a winch line and one or more light elongate members.Preferably the system includes one or more buoyant elongate members.

The A/R system preferably includes a connector for connecting the winchline releasably to the one or more buoyant elongate members. Theconnector preferably includes a stored energy arrangement for providingenergy to actuate the release of the winch line from the one or moreelongate members. The stored energy arrangement may take various formsincluding an electrical battery but preferably the energy is stored aspressurised hydraulic fluid in an accumulator and preferably the releaseof the winch line is hydraulically actuated. The hydraulic actuation mayserve to extend or retract radially extending parts of a male elementfitted inside a female element and unable to disengage when the partsare extended.

A joint may be provided for connecting the one or more buoyant elongatemembers to a pipeline. The joint preferably allows pivotal movement ofthe one or more buoyant elongate members about an axis transverse to thelongitudinal axis of the end of the pipeline. Furthermore the jointpreferably also allows pivoting of the one or more buoyant elongatemembers about the longitudinal axis of the end of the pipeline. Byallowing such pivoting it becomes possible to arrange for the one ormore buoyant elongate members to pivot upwardly from the part of thesealine resting on the seabed and therefore to enable a connectionbetween the winch line and the buoyant members to be well clear of theseabed.

The invention still further provides a pipe laying vessel including anA/R system as defined in any of the paragraphs above. The vesselpreferably includes a tensioning arrangement for withstanding thetension exerted by a pipeline during laying and a winch for use whenabandoning the pipeline during laying, wherein the safe working load ofthe winch is lower than the safe working load of the tensioningarrangement.

The invention also provides a pipe laying vessel including an A/R systemand comprising:

a tensioning arrangement for withstanding the tension exerted by apipeline during laying,

a winch and a winch line,

wherein the safe working load of the winch is lower than the safeworking load of the tensioning arrangement. Such a vessel may have anyof the features referred to in the paragraphs above.

With a conventional winch system it would be undesirable to provide anabandonment/recovery system that was unable to cope in all theconditions that might be encountered because the SWL of the winch wasnot great enough. In the present invention, however, that reasoning isdisregarded and a novel approach adopted.

The method and system employed in the preferred embodiments of theinvention for disconnecting the winch line from the sealine is itselfnovel and may be applied in other situations apart from those referredto above. Thus according to a third aspect of the invention there isprovided a method for disconnecting a winch line from a pipeline, themethod including the steps of:

connecting a first part of a connector to the pipeline,

providing a second part of the connector on the winch line, the firstand second parts being able to mate with one another and being providedwith a locking/unlocking arrangement that can be remotely actuated,

mating the first and second parts of the connector and locking themtogether, and

when the first and second parts of the connector are underwater,remotely actuating the locking/unlocking arrangement to unlock the partsand thereafter disconnecting the parts.

Also according to the third aspect of the invention there is provided adisconnection system for disconnecting a winch line from a pipeline, thesystem comprising: a first part of a connector, the first part beingconnectable to a pipeline; and a second part of the connector, thesecond part being connectable to a winch line; the first and secondparts being able to be connected and locked together by a lockingmechanism and being able to be unlocked when underwater by remoteactuation of the locking mechanism.

The method and system according to the third aspect of the inventionmay, except where inappropriate, include any of the features referred toabove with respect to the first and second aspects of the invention. Forexample, the remote actuation may be automatic at a certain angle, maybe initiated by a physical contact by an ROV or may be initiated by anacoustic signal. In an embodiment of the invention described below, thefirst and second parts of the connector include a male part and a femalepart, the male part being accommodated by the female part when the firstand second parts are connected. In that embodiment, the first part formsthe female part and the second part forms the male part (although itwill be appreciated that the first part could be the male part and thesecond part could be the female part). Thus a significant proportion ofone of the first and second parts may be inserted into and accommodatedby the other. In particular, in the case where a stored energyarrangement is provided, it is preferred that that arrangement be housedin a section of one of the first and second parts that is, when theparts are connected, accommodated by the other of the parts, therebyenabling the disconnection system to be compact. The disconnectionsystem preferably comprises a plurality of radially movable lockingmembers being movable to and from positions in which the members lockthe first and second parts together. The locking members may for examplebe moved, in use, by axial movement of a camming surface.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example,with reference to the drawings, of which:

FIG. 1 shows an S-lay vessel abandoning a pipeline in shallow watersaccording to the prior art;

FIG. 2 shows the vessel of FIG. 1 abandoning a pipeline and rope inshallow waters according to the prior art;

FIG. 3 shows an A/R system embodying the invention, shortly after theabandonment process has been initiated and before the end of thepipeline to be abandoned has reached the seabed;

FIGS. 4a to 4 c show progressive stages in the process of abandoning thepipeline;

FIG. 5a shows the arrangement of the system when recovery of thepipeline is just about to begin;

FIG. 5b shows the next stage in the recovery process;

FIGS. 6a to 6 c show progressive stages, comparable to FIGS. 4a to 4 c,in a slightly modified process of abandoning the pipeline;

FIG. 7 is a sectional view of an intermediate string for use in thedescribed embodiment;

FIG. 8a is a sectional view of a seabed joint connected to one end ofthe intermediate string;

FIG. 8b is a sectional view along the lines b—b in FIG. 8a;

FIG. 9 is a sectional view of an abandonment string for use in thedescribed embodiment;

FIG. 10 is a sectional view of a female part of a connector fixed to anend of the abandonment string of FIG. 9;

FIG. 11a is a sectional view of a male part of the connector whosefemale part is shown in FIG. 10, with the connector shown in a released(retracted) state; and

FIG. 11b is a sectional view in a plane perpendicular to the plane ofthe section of FIG. 11a of the male part shown in FIG. 11a, with theconnector shown in a locking (extended) state.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is convenient first to describe briefly a conventional A/R method andsystem of the kind used when laying a pipeline in water that is notdeep. FIGS. 1 and 2 illustrate such a method. In FIG. 1 a vessel 1 isshown that has been laying a pipeline 2 on the seabed 3 but hasinterrupted laying, for example because of bad weather. At a stage whenthe end of the pipeline 2 was about to leave the vessel, anabandonment/recovery head 4 was welded to the end of the pipeline and asteel rope 5 connected to the head 4. The other end of the steel rope 5is mounted on a winch (not shown) on the vessel 1.

After connection of the rope, tension in the sealine is maintained bythe winch rather than the pipeline laying and tensioning mechanismprovided on the vessel for use during normal laying. The vesselcontinues laying the end portion of the pipeline 2 as the steel rope 5is let out from the winch under tension. Once the head 4 has reached theseabed (the stage shown in FIG. 1), the tension in the rope 5 can bereleased. In reasonable conditions, the rope 5 remains connected to thewinch on the vessel 1, enabling the pipeline to be recovered relativelyquickly and simply by reversing the operations described above.

In very poor weather conditions or other extreme circumstances thevessel 1 completely abandons the rope 5 as shown in FIG. 2. The rope 5lies substantially flat on the seabed, as does the pipeline 2. The rope5 is connected, at the end which is not connected to the A/R head 4, toa pennant line 6, which is in turn connected to a marker buoy 7. Theconnection of the pennant line 6 to the rope 5 can be carried out on thevessel.

The vessel 1 shown in FIGS. 1 and 2 is an S-lay vessel; that is, thepipeline 2 generally has the form of an elongated “S” during the layingprocedure. Such an S-lay vessel is most suitable for laying a pipelinein relatively shallow water.

The A/R system and method that will now be described with reference tothe remaining drawings is especially suited for the case where apipeline is being laid in deep water using the J-lay technique, in whichthe pipeline generally has the form of a “J” during the layingprocedure.

In order to aid understanding of the new A/R system and method it willfirst be described in overview and, thereafter, details of particularparts of the system and method will be provided.

The A/R method and system is illustrated in FIGS. 3 to 5. At thecommencement of the abandonment procedure the pipeline 2 is being laid,using a J-lay technique, by the vessel and the end of the pipeline is atthe vessel, indicated by reference numeral 10 and is being allowed topass down a tower 10A of the vessel. A tensioning arrangement of a kindknown per se is provided on the tower 10A; as the pipeline is laid, solengths of pipe are added to the end of the pipeline in a manner knownper se. One suitable form of vessel and tower arrangement is describedin our application No. PCT/EP99/05201.

At the stage where it is decided to abandon the pipeline, the pipelaying operation is changed: instead of welding a further length of pipeto the end of the pipeline 2, an intermediate string 11 of the samediameter as the pipe is welded to the end of the pipeline 2. In aparticular example the intermediate string has a length of about 48 mand a diameter of about 60 cm; it is made of steel and sealed at eachend.

The intermediate string 11 is passed down the tower 10A using thetensioning arrangement on the tower in the same way as an ordinarylength of pipe.

One end of an abandonment string 13 is connected to the upper end of theintermediate string 11; in this case, however, in place of a weldedconnection, a special joint 12 (referred to herein as a seabed joint) isprovided between the intermediate string 11 and the abandonment string13. The seabed joint 12 allows rotation of the adjoining end of theabandonment string about the longitudinal axis of the intermediatestring 11 and also allows pivoting of the abandonment string 13 about anaxis perpendicular to its longitudinal axis.

The abandonment string 13 is of the same diameter as the pipe formingthe pipeline 2 and in the particular example described has a length ofabout 50 m and a diameter of about 60 cm. The abandonment string is madeprincipally of a glass fibre reinforced vinyl ester resin, is sealed atboth ends, and is highly buoyant in water.

The abandonment string 13 is also passed down the tower 10A using thetensioning arrangement on the tower in the same way as an ordinarylength of pipe.

As the abandonment string 13 is lowered into the water it is pulled downby the intermediate string 11, overcoming the upward buoyancy force dueto the submerging of the string 13. As a result the tension in thesealine is at a maximum at the joint between the intermediate string 11and the buoyant abandonment string 13 and reduces upwardly along thelength of the abandonment string 13 to a minimum at sea level. Thereforeas the abandonment string 13 is lowered into the water the tension thathas to be accommodated in the tower 10A reduces.

A connector 14 (referred to herein as an A/R connector) is provided atthe upper end of the abandonment string 13 and one end of a steel rope16 is connected, via the A/R connector, to the abandonment string 13.The steel rope is connected to a winch (not shown) on the vessel.

Once the upper end of the abandonment string 13 has reached the lowerend of the tower 10A and the rope 16 has been connected, the tension inthe sealine is transferred from the tensioning arrangement on the tower10A to the winch. It should be noted that the tension that the winch hasto withstand is less than the tension that was accommodated duringlaying of the pipeline by the tower 10A and also less than the tensionin the pipeline 2 at its end connected to the intermediate string 11.

The steel rope 16 is then let out from the winch lowering the pipeline 2and strings 11, 13 towards the seabed, to the position shown in FIG. 3(where the lowermost part of the pipeline is merely represented by adotted line).

The strings 2, 11, 13 are lowered to the seabed by the A/R winch.

As previously described, the intermediate steel string 11 and theabandonment string 13 are free to rotate and pivot relative to oneanother about the joint 12 so that, regardless of the orientation of thestring 11 on the seabed, the axis of pivoting of the string 13 canadjust itself to be horizontal. FIG. 4a shows the situation afterfurther letting out of the steel rope 16 from the winch: the pipeline 2and intermediate string 11 are lying flat on the seabed 3 and thebuoyant abandonment string 13 has pivoted about joint 12 so that it issubstantially vertical. The A/R winch then lowers the steel rope 16further so that the abandonment string 13 becomes inclined to thevertical as shown in FIG. 4b. When the abandonment string 13 is inclinedat about 60° to the vertical, the A/R connector 14 automaticallydisconnects the rope 16 from the string 13 and the rope is then winchedback to the vessel 10. The abandonment string 13 is then free and pivotsabout joint 12 back to the vertical under its own buoyancy to theposition shown in FIG. 4c.

When conditions permit recovery of the pipeline 2, connection of thestring 13 via the A/R connector 14 to the rope 16 can be performed bymanoeuvring the vessel 10 to the required position and using an ROV 20as shown in FIG. 5a. The rope 16 can then be guided to the A/R connector14 with the help of an acoustic camera or sonar and the connectionre-established following an actuation signal from the vessel 10 or by aphysical manipulation by the ROV 20; for example, a manipulator of theROV can operate a ball valve, installed in a dedicated ROV panel on theupper part of the A/R connector 14, as will be described further below.FIG. 5b shows the situation after the connection has been made.

As will be appreciated, in order to complete recovery of the end of thepipeline, the various operations described above in respect of loweringof the pipeline to the seabed are reversed. Thus, the winch is used tohaul in the steel rope 16 until the A/R connector 14 reaches the tower10A of the vessel; tension in the string 13 is then transferred from thewinch to the tower 10A; the rope 16 disconnected; the string 13 raisedup the tower and, once the string 11 reaches the tower 10A, the string13 is disconnected from the string 11; the string 11 is then raised upthe tower until the upper end of the pipeline 2 reaches the tower 10A;the string 11 is then cut from the pipeline 2; thereafter laying of thepipeline can begin again.

FIGS. 6a to 6 c show a modification that can be made to the proceduredescribed above. In that case, instead of there being one length ofabandonment string 13 there are several lengths (in the particularexample shown, three lengths). That enables the tension at the upper endof the sealine to be reduced further before the tension is borne by therope 16 rather than by the tower 10A. Such an arrangement can be usefulwhen operating in very deep water assuming that the part of the pipelineextending between the vessel and the seabed is flooded; in that case thetension at the upper end of the pipeline 2 at the beginning of theabandonment process will be higher and therefore more than oneabandonment string 13 may be required to reduce the tension to a levelthat the winch can accommodate.

A quick connector 30 can be used to make the connections between theabandonment strings 13. Such connectors, enabling connections to be madebetween strings at a welding station on the tower 10A are availablecommercially. An example is the Merlin connector of Oil StatesIndustries (UK) Ltd.

Another modification to the procedure described above to reduce thetension that the winch has to accommodate in the case of a floodedpipeline is to connect a number of lengths 40 of sealed empty pipelineto the end of the pipeline 2, prior to connecting the intermediatestring 11. Such a procedure can be adopted instead of or as well as theuse of a plurality of abandonment strings 13. Such lengths of sealedempty pipeline are preferably provided at each end with valves to allowfluid flow along those lengths if or when desired.

Having now provided an overview of the system and a description of theprocedure adopted, particular examples of designs for component parts ofthe system will be described.

FIG. 7 shows the construction of the intermediate string 11 referred toabove. At the left hand side of the drawing, the end of the pipeline 2is shown in dotted outline. The string 11 is formed principally of amain central section 111 and has two end sections 112 and 115. The endsection 112 is connected by welds 113 to the pipeline 2 and to thecentral section 111; the end section 112 incorporates a hemisphericalsection 114 which provides a seal between the pipeline 2 and theinterior of the string 11. Similarly the end section 115 is connected bywelds 116 to a first part 121 of the joint 12 and incorporates ahemispherical section 117 which provides a seal between the interior ofthe string 11 and the unsealed joint 12.

FIGS. 8a and 8 b show the construction of the joint 12. The first part121 is hollow of generally circular cross-section and houses acylindrical part 122 of a connector element 123 that is rotatablymounted in the part 121 for rotation about the central longitudinal axisof the intermediate string 11. The connector element 123 has, at theopposite end to the cylindrical part 122, a central pivot mounting 124having a transverse through bore 125 (FIG. 7). A pair of forked arms 126of a third part 127 of the connector extend on either side of the pivotmounting 124 and have transverse through bores aligned with the bore inthe mounting 124. A pin 128 passes through the aligned through bores andprovides a pivot mounting of the third part 127 relative to theconnector element 123.

When in use the intermediate string 11 is resting on the seabed, thethird part 127 (to which the abandonment string 13 is connected, as willbe described below) is able to pivot relative to the connector element123 from a position in which the string is at an angle of 60° to thevertical (the position shown in FIG. 4b and one of the positions shownin dotted outline in FIG. 8a) through a vertical position to a positioninclined backwardly at an angle of 25° to the vertical (the other of theextreme positions shown in dotted outline in FIG. 8a).

The abandonment string 13 is shown in FIG. 9. It generally comprises aseries of tubular members 131 joined together at welded butt strapjoints 132. In the example shown four tubular members 131 make up oneabandonment string 13. The third part 127 of the seabed joint terminatesin an annular portion 129 which is welded to a matching annular portion133 of a steel insert provided at the adjoining end of the abandonmentstring 13. The steel insert 133 is fixed in the member 131 at the timethat the string is formed and has annular grooves at the interface withthe glass reinforced resin of the member 131 so that a pressureresistant watertight and strong mechanical connection is formed betweenthe steel insert and the member 131.

At the other end of the abandonment string a similar steel insert 134 isprovided fixed inside its respective tubular member 131 in the same wayas the insert 133.

The insert 134 terminates in a tubular portion 135 which can be weldedto a corresponding portion 140 of a female part 141 of the A/R connector14 as shown in FIG. 10.

Referring now to FIGS. 10, 11 a and 11 b, the structure and operation ofthe A/R connector 14 will be described. The female part 141 generallycomprises a hollow cylindrical cavity 142 of circular cross-sectionformed with an inwardly projecting lip 143 around an open end 144. Aseries of six pads 145 are equiangularly spaced around the interior ofthe cavity to provide guide surfaces for receiving a male part of theconnector, as will be described below.

FIGS. 11a and 11 b show the male part 171 of the connector 14 insection, with the female part 141 and the end of the abandonment string13 being shown in dotted outline. It will be seen that the male part 171has a main body part 172 shaped to fit within the female part 141 with afrustoconical leading end to assist entry into the female part. Aperipheral flange 173 on the body part limits the penetration of themale part 171 into the female part 141. It will also be seen that themain body part 172 is a close fit within the pads 145. Six lockingmembers 174 are mounted in the main body part 172 and are equiangularlyspaced around the body part. The locking members 174 are each movableradially between a restricted position shown in FIG. 11a where theradially outermost surface of each member is flush with acircumferential surface 175 of the main body part, and an extendedposition shown in FIG. 11b where the radially outermost surface of eachmember projects outwardly against the interior of a peripheral wall 146of the female part 141. As can be seen in FIGS. 11a and 11 b, when themembers 174 are in the retracted position shown in FIG. 11a, the malepart 171 of the connector can move freely in and out of the female part141, but when the members 174 are in the extended position, the malepart 171 is held within the female part 141 by the engagement of themembers 174 with the lip 143 of the female part 141.

Radial movement of the locking members 174 is controlled by axialmovement of a wedge member 176 which cammingly engages sloping rearsurfaces on the locking members 174 to extend the members and also haskeyways in which projecting parts of the locking members 174 engage toretract the members. Thus, movement of the wedge member 176 along thelongitudinal axis of the connector to the right as shown in FIGS. 11aand 11 b serves to extend the members 174 and movement to the leftserves to retract them. The wedge member 176 is axially mounted by aguide rod 177 engaging a central bore in the main body part 172 and itsmovement is controlled by a double acting hydraulic cylinder 178pressurised by a nitrogen pre-charged accumulator 179. An oil dischargetank 180 is also provided. In the particular example described thehydraulic circuit is arranged so that actuation to cause the cylinder178 to extend the locking members is achieved by contact of amanipulator on the ROV 20 to actuate a ball valve (not shown) in thevicinity of the flange 173 of the male part 171 of the connector 14,whilst actuation to cause the cylinder 178 to retract the lockingmembers is achieved by contact of a connector part 15 connected to theend of the rope 16 as will now be further described.

The connector part 15 is shown in dotted outline in FIGS. 11a and 11 b.It has a main body part 151 at one end of which there is an eye 152, towhich the steel rope 16 (not shown) is connected and at the opposite endof which a coupling part 153 connected to the male part 171 of theconnector 14 is provided. For this purpose the male part 171 has a pairof forked arms 180 between which the coupling part 153 is received, asbest seen in FIG. 11a. The forked arms 180 and the coupling part 153have aligned bores through which a pin 154 passes connecting togetherthe connector part 15 and the male part 171, whilst allowing pivotalmovement of one part relative to the other.

The coupling part 153 is formed with a pair of lugs 155. When the steelrope 16 and the abandonment string 13 are aligned, the parts are in therelative positions shown in FIG. 11b, but as the rope 16 becomesinclined to the string 13 as shown in FIG. 4b, so the coupling part 153pivots relative to the male part 171 and, in the example illustrated,after pivoting through 60°, one of the lugs 155 comes into contact withand passes against a spring loaded actuator plate 156. Depressing theactuator plate 156 against its spring bias serves to actuate a ballvalve controlling the hydraulic cylinder 178 to retract the lockingmembers 174, leaving the male and female parts of the connector 14 freeto separate from one another.

In the event that the connector 14 does not operate properly theintermediate string 11 on the seabed can be cut using an ROV, anotherconnection made to the cut end of the intermediate string 11 and thepipeline thereafter recovered.

In the embodiment of the invention described above the abandonmentstring is made from tubular members of glass fibre reinforced vinylester resin. Other materials that may be used include other compositematerials such as kevlar, carbon, titanium, aluminium or anothermaterial having a high tensile strength and low density. The axial forcedue to the weight of the pipeline in the water and the external pressuredue to the water depth at various stages of the operation need to betaken into account when choosing a material and a design for theabandonment string 13.

If desired, a swivel may be incorporated in the connection of the steelrope 16 to the coupling part 153 of the connector part 15.

In the case of the A/R connector described in detail above withreference to FIGS. 10, 11 a and 11 b, actuation of the cylinder 178 isachieved by a mechanical actuation generated either automatically bypivoting of the coupling part 153 or by a manipulator arm of an ROV.Other forms of remote actuation may be provided, either in place of orin addition to those described; for example actuation may be effected byan acoustic signal from the vessel 10.

The intermediate string 11 may be provided with valves in thehemispherical sections 114, 117 to allow liquid flow along the stringand/or with one or more valves to allow seawater surrounding the sealineto enter the hollow interior of the string 11. It is advantageous toprovide such valves so that the intermediate string can be flooded andpressures equalised in the event that the A/R system does not operateand the intermediate string is to be cut.

The A/R method and system described above provide numerous advantages.Firstly it is possible to employ a winch and rope whose load bearingcapacities are substantially less than the tension in the pipelineduring laying. It therefore becomes viable to perform theabandonment/recovery operation even in deep water. Also the buoyantnature of the abandonment string 13 means that connections anddisconnections between the steel rope 16 and the string 13 are made at alocation well clear of the seabed, thereby facilitating thoseoperations. Furthermore, the nature of the connections between thepipelines 2 and the intermediate string 11, between the intermediatestring 11 and the abandonment string 13, and between the abandonmentstring 13 and the rope 16 are such that they can be made and unmaderelatively simply and quickly. With a system of the kind described aboveit is practical to abandon and recover a pipeline of 60 cm diameter at awater depth of 2200 m using a winch with a safe working load (SWL) of5500 kN and a steel rope of diameter 120 mm.

What is claimed is:
 1. A method of abandoning a pipeline being J-laid bya vessel, wherein a sealine comprising the pipeline is initially held bya pipe laying and tensioning arrangement, the method including the stepsof reducing the tension at sea level of the sealine, and thereafterlowering the end of the sealine towards the seabed using a line from awinch on the vessel, thereby enabling the maximum load bearing capacityof the winch to be substantially less than the maximum load bearingcapacity of the pipe laying and tensioning arrangement.
 2. A methodaccording to claim 1, in which the step of reducing the tension at sealevel of the sealine being laid by the vessel comprises connecting oneor more light elongate members to the end of the pipeline and loweringthe elongate members into the sea.
 3. A method according to claim 2, inwhich at least one of the elongate members is buoyant.
 4. A methodaccording to claim 3, in which said at least one elongate member iscomposed principally of a material selected from the group consisting ofglass fibre reinforced resins, composite materials, carbon, titanium andaluminum.
 5. A method according to claim 4, in which said at least oneelongate member is composed principally of a glass fibre reinforcedvinyl ester resin.
 6. A method according to claim 3, in which anintermediate elongate member is connected between the end of thepipeline and the buoyant elongate member.
 7. A method according to claim6, in which the intermediate elongate member is hollow and is sealedagainst the ingress of water.
 8. A method according to claim 7, in whichthe intermediate elongate member includes a valve to enable the interiorof the hollow elongate member to be flooded by sea water.
 9. A methodaccording to claim 6, further including the step of cutting theintermediate elongate member, connecting the winch line to the end ofthe elongate member that is connected to the pipeline and recovering theend of the pipeline.
 10. A method according to claim 3, in which said atleast one elongate member is connected to the pipeline through aconnection which allows pivoting of the elongate member about ahorizontal axis when the pipeline is lying horizontally.
 11. A methodaccording to claim 2, in which the one or more elongate members arelowered using the pipe laying and tensioning arrangement on the vesselthat is used for laying the pipeline.
 12. A method according to claim11, in which the load bearing capacity of the winch is substantiallyless than the load bearing capacity of the tensioning arrangement.
 13. Amethod according to claim 1, further including the subsequent step ofdisconnecting the sealine from the winch line.
 14. A method according toclaim 13, in which an ROV is provided and the disconnection of thesealine from the winch line is initiated by physical contact of the ROVwith a connector connecting the sealine to the winch line.
 15. A methodaccording to claim 13, in which the disconnection of the sealine fromthe winch line is initiated remotely.
 16. A method according to claim15, in which the disconnection of the sealine from the winch line iseffected by a signal transmitted remotely from an ROV or the vessel. 17.A method according to claim 16, in which the signal is an acousticsignal.
 18. A method according to claim 15, in which the disconnectiontakes place automatically at a predetermined angle of the adjoining endsof the winch line and the sealine.
 19. A method according to claim 18,in which the predetermined angle is in the range of 45° to 75°.
 20. Amethod according to claim 13, in which the energy required for thedisconnection is stored in the connecting arrangement.
 21. A methodaccording to claim 13, in which after disconnection of the sealine fromthe winch line an end portion of the sealine projects upwards from theseabed.
 22. A method according to claim 1, further including recoveringthe abandoned pipeline by carrying out the further steps of raising theend of the sealine to the vessel using the winch, and raising thesealine further using the pipe laying and tensioning arrangement on thevessel that is used for laying the pipeline.
 23. A method according toclaim 22, in which the further steps include a preliminary step ofconnecting the winch line to the sealine.
 24. A method of abandoning apipeline being laid by a vessel, including the steps of connecting oneor more buoyant elongate members to the end of the pipeline, thereafterlowering the elongate members into the sea and then abandoning thepipeline, wherein one or more elongate members are so connected to thepipeline that the end of the elongate member at the end of the pipelineprojects upwardly from the seabed, thereby facilitating the futurerecovery of the pipeline.
 25. A method for disconnecting a winch linefrom a pipeline, the method including the steps of: connecting a firstpart of a connector to the pipeline, providing a second part of theconnector on the winch line, the first and second parts being able tomate with one another and being provided with a locking/unlockingarrangement that can be remotely actuated, mating the first and secondparts of the connector and locking them together, and when the first andsecond parts of the connector are underwater, remotely actuating thelocking/unlocking arrangement to unlock the parts and thereafterdisconnecting the parts.
 26. A disconnection system for disconnecting awinch line from a pipeline, the system comprising: first and secondparts of a connector and a locking mechanism, wherein the first part isconnectable to a pipeline, the second part is connectable to a winchline, the first and second parts are able to be connected and lockedtogether by the locking mechanism, and the first and second parts areable to be unlocked when underwater by remote actuation of the lockingmechanism.
 27. A pipe laying vessel including a pipe laying andtensioning arrangement for withstanding the tension exerted by apipeline during laying of the pipeline and an A/R system comprising anA/R winch and line, the winch and line being suitable for use whenabandoning a pipeline during laying, and one or more light elongatemembers, wherein the light elongate members are configured forconnecting to a pipeline for the purpose of reducing the tension at sealevel of a sealine comprising or consisting of the pipeline, and thesafe working load of the winch is lower than the safe working load ofthe pipe laying and tensioning arrangement.
 28. A method of abandoning apipeline being laid by a vessel, wherein a sealine comprising thepipeline is initially held by a pipe laying and tensioning arrangementused for laying the pipeline, the tension at sea level of the sealinebeing at a first level, the method including the steps of (i) connectingone or more buoyant elongate members to the end of the pipeline, (ii)using the pipe laying and tensioning arrangement to lower the elongatemembers into the sea, thereby reducing the tension at sea level of thesealine, (iii) thereafter holding the sealine with the pipe laying andtensioning arrangement with the tension at sea level of the sealinebeing at a second level substantially lower than the first level, (iv)thereafter lowering the end of the sealine towards the seabed using aline from a winch on the vessel, and (v) thereafter disconnecting thesealine from the winch line wherein the maximum load bearing capacity ofthe winch is substantially less than the maximum load bearing capacityof the pipe laying and tensioning arrangement.
 29. A method ofabandoning a pipeline being laid by a vessel, wherein a sealinecomprising the pipeline is initially held by a pipe laying andtensioning arrangement, the method including the steps of (i) reducingthe tension at sea level of the sealine by (a) connecting one or morebuoyant members to the end of the pipeline, (b) connecting anintermediate elongate member between the end of the pipeline and abuoyant elongate member, and (c) lowering the elongate members into thesea, (ii) thereafter lowering the end of the sealine towards the seabedusing a line from a winch on the vessel, the method thereby enabling themaximum load bearing capacity of the winch to be substantially less thanthe maximum load bearing capacity of the pipe laying and tensioningarrangement.
 30. An A/R system including a winch and line and one ormore light elongate members, wherein the A/R system is adapted forcarrying out a method of abandoning a pipeline on a vessel having a pipelaying and tensioning arrangement, and the one or more light elongatemembers are so shaped and configured (i) to be connectable to the end ofthe pipeline held by the pipe laying and tensioning arrangement to forma sealine, and (ii) to be able to be lowered into the sea by the pipelaying and tensioning arrangement, so that the tension at sea level ofthe sealine may be reduced before connecting the line of the winch tothe pipeline via the one or more light elongate members, whereby themaximum load bearing capacity of the winch may be substantially lessthan the maximum load bearing capacity of the pipe laying and tensioningarrangement.
 31. An A/R system including a winch and line, one or morebuoyant elongate members and an intermediate elongate member, whereinthe A/R system is arranged to be suitable for carrying out a method ofabandoning a pipeline on a vessel having a pipe laying and tensioningarrangement for laying and holding a pipeline, the one or more buoyantelongate members are so shaped and configured to be connectable betweenthe end of the pipeline and a buoyant elongate member, the intermediateelongate member is so shaped and configured to be connectable to the endof a sealine comprising the pipeline, and the elongate members are soshaped and configured to be able to be lowered into the sea by the pipelaying and tensioning arrangement, so that the tension at sea level ofthe sealine may be reduced before connecting the line of the winch tothe pipeline via the one or more elongate members, whereby the maximumload bearing capacity of the winch may be substantially less than themaximum load bearing capacity of the pipe laying and tensioningarrangement.
 32. An A/R system including a winch and line and one ormore light elongate members, wherein the A/R system is arranged to besuitable for carrying out a method of abandoning a pipeline on a vessel,and the one or more light elongate members are so shaped and configuredto be connectable to the end of the pipeline so that after abandonmentof the pipeline the end of the elongate member at the end of thepipeline projects upwardly from the seabed, thereby facilitating thefuture recovery of the pipeline.
 33. An A/R system including a winch andline and at least one buoyant elongate member, wherein the A/R system isarranged to be suitable for carrying out a method of abandoning apipeline on a vessel, and the buoyant elongate member includes a pivotjoint that facilitates pivoting movement of the end of the buoyantelongate member about a position at or near the end of a sealinecomprising the pipeline, the buoyant elongate member being buoyantenough that after abandonment of the pipeline the end of the elongatemember at the end of the pipeline projects upwardly from the seabed,thereby facilitating the future recovery of the pipeline.
 34. An A/Rsystem according to any of claims 30 to 33 including a pipe-laying andtensioning arrangement, wherein the maximum load bearing capacity of thewinch is substantially less than the maximum load bearing capacity ofthe pipe laying and tensioning arrangement.
 35. An A/R system accordingto claim 30 including one or more buoyant elongate members.
 36. An A/Rsystem according to claim 31 or 35, including a joint for connecting theone or more buoyant elongate members to the pipeline.
 37. An A/R systemaccording to claim 36, in which the joint allows pivotal movement of theone or more buoyant elongate members about an axis transverse to thelongitudinal axis of the end of the pipeline.
 38. An A/R systemaccording to claim 37, in which the joint also allows pivoting of theone or more buoyant elongate members about the longitudinal axis of theend of the pipeline.
 39. An A/R system according to any of claims 31, 33and 35, including a connector for connecting the winch line releasablyto the one or more buoyant elongate members.
 40. An A/R system accordingto claim 39, in which the connector includes a stored energy arrangementfor providing energy to actuate the release of the winch line from theone or more buoyant elongate members.
 41. An A/R system according toclaim 40, in which the release of the winch line is hydraulicallyactuated.
 42. A pipe laying vessel including a pipe laying andtensioning arrangement for withstanding the tension exerted by apipeline during laying of the pipeline and an A/R system comprising anA/R winch and line, the winch and line being suitable for use whenabandoning a pipeline during laying, and one or more buoyant elongatemembers, wherein the buoyant elongate members are configured forconnecting to the end of a pipeline for the purpose of reducing thetension at sea level of a sealine comprising the pipeline, the one ormore buoyant elongate members and pipe laying and tensioning arrangementare so shaped and configured that the pipe laying and tensioningarrangement can hold and lower the sealine by holding a buoyant elongatemember at the end of the sealine, and the maximum load bearing capacityof the winch is substantially less than the maximum load bearingcapacity of the pipe laying and tensioning arrangement.
 43. A pipelaying vessel including a pipe laying and tensioning arrangement forwithstanding the tension exerted by a pipeline during laying of thepipeline and an A/R system according to any one of claims 30 to
 33. 44.A disconnection system for disconnecting a winch line from a pipeline,the system comprising: a first part of a connector, the first part beingconnectable to a pipeline; and a second part of the connector, thesecond part being connectable to a winch line; the first and secondparts being able to be connected and locked together by a lockingmechanism and being able to be unlocked when underwater by remoteactuation of the locking mechanism, wherein the system is so arrangedthat upon unlocking, the first part of the connector remains attached tothe pipeline and the second part of the connector attached to the winchline.
 45. A method of abandoning a pipeline being laid by a vessel,wherein a sealine comprising the pipeline is initially held by a pipelaying and tensioning arrangement, the tension at sea level of thesealine being at a first level, the method including the steps of: (i)reducing the tension at sea level of the sealine, (ii) thereafterholding the sealine with the pipe laying and tensioning arrangement withthe tension at sea level of the sealine being at a second levelsubstantially lower than the first level, and (iii) thereafter loweringthe end of the sealine towards the seabed using a line from a winch onthe vessel, whereby the maximum load bearing capacity of the winch isable to be substantially less than the maximum load bearing capacity ofthe pipe laying and tensioning arrangement.